US3640597A

US3640597A - Method of producing neutron source tube with coated target

Info

Publication number: US3640597A
Application number: US336075A
Authority: US
Inventors: Lowell A Noble
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1964-01-06
Filing date: 1964-01-06
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08

Abstract

A method of making an active metal target for a neutron source tube. Deuterium or tritium gas is sorbed into the active metal. The target is then heated and reacted with a coating gas to form a protective coating over the target which prevents desorbtion of the deuterium or tritium during operation of the tube.

Description

0 United States Patent 1151 3,640,597 Noble Feb. 8, 1972 [54] METHOD OF PRODUCING NEUTRON [56] Reference Cited SOURCE TUBE WITH COATED UNITED STATES PATENTS TARGET 1,909,916 5/1933 Dnffenbach et al ..l48/6.3 x [72] Inventor: Lowell A. Noble, Hillsborough, Calif, 218471331 12/1958 2,865,797 l2/l958 ....14s/20.3 [73] Ass1gnee: Varian Associates 2,870,339 H1959 .t..250/84.5 2,899,345 8/1959 ..l48/6.3 [22] Ffled' 1964 2,917,419 12 1959 Robinson ..l48/6.3 211 Appl. No.: 336,075

Primary ExaminerR0dney D. Bennett, J r. Related U.S. Application Data Assistant Examiner-Daniel C. Kaufman [62] Division of Ser. No. 371,803, June 21, 1960. Ammey Rben 57 ABSTRACT 52 u.s.c1 ..316/10, 29/2517, 148/63,

50 5 3 2 A method of making an active metal target for a neutron 51 Int. Cl ..H01j 9/38, G21 g 3 04 some tube Deuterium of tritium gas is some the active 1581 mold of Search ..250 84.5- 313/615' 316/4 metal- The target is heated and reamed with a math"; gas

to form a protective coating over the target which prevents desorbtion of the deuterium or tritium during operation of the tube.

4 Claims, 2 Drawing Figures I l 9 I 1 x 1 1 I INVENTOR LOWELL A NOBLE Wm 191a A 7' TOPNE Y METHOD OF PRODUCING NEUTRON SOURCE TUBE WITH COATED TARGET This application relates specifically to the method aspects of the invention and is a division of my copending application Ser. No. 371,803, filed June 21,1960.

This invention relates to the art of making vacuum tubes, particularly tubes which provide a source of neutrons.

In general a neutron source tube comprises a vacuumtight envelope filled with a hydrogen isotope gas (deuterium or tritium), an active metal target in the envelope loaded with a hydrogen isotope, and electrodes which can be energized to cause the gas to ionize and the ions to strike the target. When the ions strike the hydrogen isotope in the target, a wellknown nuclear reaction occurs which liberates neutrons. It is also known that the yield of neutrons is substantially more when deuterium reacts with tritium than when deuterium reacts with deuterium, or when tritium reacts with tritium. Conventionally the target is 'made of an active metal such as titanium which will sorb large quantities of hydrogen isotopes. In another form the source tube is evacuated instead of gas filled and contains a supply of hydrogen isotopes which are sorbed in an active metal and can be desorbed and ionized by a spark discharge. The ions are then attracted to the target.

One problem associated with neutron source tubes in the past was that the pressure and composition of the gas in the envelope varied during the life of the tube, causing it to give erratic results.

According to this invention it was determined that the primary reason why the pressure and composition of the gas varied was that over a period of time some of the hydrogen isotope gas would be sorbed by the target and some of the hydrogen isotope in the target would be desorbed into the gas. In the case of the modified tube hydrogen would be desorbed from both the target and the supply metal. It is true that this action does reach equilibrium at a given gas pressure, and given temperature. However, temperature does not remain constant as a practical matter, and the time required to reach equilibrium is relatively long, particularly at low temperature. Therefore, equilibrium cannot be maintained and the interchange action does occur.

As previously mentioned the greatest neutron yield occurs when unlike hydrogen isotopes react with each other. In the past it was difficult to obtain this optimum neutron yield because if the tube or the supply metal were filled with one isotope of hydrogen and the target was filled with another, the isotopes would move from one place to another so that the supply metal, target and tube would each contain a mixture of isotopes, and some deuterium-deuterium or tritium-tritium reactions would occur which would lower the neutron yield. Even when a single isotope was used throughout in an attempt to obtain a uniform predictable neutron yield, the results were erratic because the active metals would sorb and desorb the hydrogen to cause a change in the gas pressure or vacuum and therefore a change in the operating conditions of the tube.

Another feature of the invention which relates to optimum neutron yield and constant gas pressure or vacuum has to do with the fact that the neutron yield increases with increase in the ratio of hydrogen to active metal in the target. Prior to this invention it was not possible to utilize the maximum ratio because the hydrogen isotope in the target would be desorbed into the tube, particularly in the case of the vacuum-type tube. This of course reduces the ratio of hydrogen to active metal; and particularly in the case of the vacuum-type tube, causes such an increase in the pressure in the tube that the tube becomes inoperable.

Thus, an object of this invention is to provide an improved neutron source tube capable of predictable high neutron yield.

Another object of this invention is to provide a neutron source tube which can be stored for long periods of time without detrimental effect.

A further object of the invention is to inhibit the sorption and desorption of hydrogen by the active metal in a neutron source tube.

By way of brief description the invention involves placing on the active metal in the tube a coating which can, in the case of the supply metal, be removed to allow desorption, and which can, in the case of the target, be penetrated by hydrogen ions.

The invention contains other objects and features of advantage some of which, with the foregoing, will be set forth in the following description of the invention. The invention is not limited to the disclosed embodiment, as variant embodiments thereof are contemplated and may be adopted within the scope of the claims.

Referring to the drawing:

' FIG. 1 is an axial cross section ofa gastype neutron source tube.

FIG. 2 is a cross section of an alternate embodiment of one end of the tube in FIG. 1 whereby the structure becomes a vacuum-type neutron source tube.

Referring to the drawing in detail FIG. 1 shows a gastight envelope comprising two cup-shaped electrodes 10 and 11 disposed at opposite ends, with a tubular ceramic insulator 12 between the two. A flange 13 on the rim of the cup-shaped electrode 10 is sealed Vacuumtight to one end of the ceramic 12 with ceramic backing ring 14 sealed to the flange 13 opposite ceramic 12 to reinforce the bond between metal flange l3 and ceramic cylinder 12.

The other end of cylinder 12 is sealed Vacuumtight to the electrode 11 with the aid of a metal sealing ring 16 which is are welded to the rim of the electrode 11. The vacuumtight seal between the ring 16 and the ceramic cylinder 12 also employs a ceramic backing ring 17 to strengthen the seal.

A neutron source tube has a target which in this tube is the electrode 11 on which a layer 18 of an active metal is disposed on the inner surface as shown, Titanium is the preferred active metal, but of course the layer 18 may be made of other active metals, such as zirconium, hafnium, lithium, lanthanum, yttrium, or thorium. A hydrogen isotope, either deuterium or tritium is sorbed by the active metal in accordance with a standard process such as first baking-out and outgassing the tube by heating the tube and forming a vacuum therein by fixing a suitable vacuum pump (not shown) to a tubulation 19 which communicates with the interior of the tube through the electrode l0, and then supplying hydrogen isotope gas into the tube through the tubulation as the tube is cooled. There are other processes for sorbing hydrogen within an active metal and the invention is not limited to the particular process described.

After the tube has been outgassed and the active metal has been loaded with a hydrogen isotope, the standard practice was then to obtain the desired pressure of hydrogen isotope gas in the tube and finally seal the tube by pinching off tubulation 19.

According to this invention, however, after the optimum amount of hydrogen isotope is sorbed by the titanium 18, the hydrogen is removed through the tubulation l9 and oxygen is supplied through the tubulation at a pressure preferably between 200 and 400 millimeters of mercury for about l hour duration at a temperature range of l50200 C. These parameters produce an oxide coating or layer 21 on the titanium which is impervious to hydrogen isotope molecules because the oxide forms a tight and adherent coating on the titanium. Other gases such as nitrogen, chlorine, carbon vapor, or gases containing carbon can be substituted for oxygen. The main requirement is that the gases must react with the active metal to form the impervious thin coating at a temperature of about 200 C. or under so that the hydrogen isotope is not desorbed from the active metal during the formation of the impervious coating. After the loaded target has thus been protected by the impervious coating, the oxygen is removed through tubulation 19, the desired hydrogen isotope gas is supplied through the tubulation, and the tube is sealed by pinching-off" the tubulation 19.

In order to operate the tube of FIG. 1, an external power source (not shown) is connected to electrodes 10 and II to form a potential gradient between them which will cause the hydrogen isotope gas to ionize. The target electrode I1 is made negative with respect to electrode so that the positive ions will strike the target. When the ions strike the target they will penetrate the coating 21 and will strike the hydrogen isotope in the active metal 18 to cause the neutron yielding reaction previously described. The coating 21 should not be made too thick because the energy required to force ions through the coating will increase as the thickness of the coating increases.

Thus it will be understood that the invention provides a coating on the active metal which prevents desorption and further sorption of hydrogen isotopes by the active metal while at the same time being penetrable by hydrogen isotope ions. In this way it is possible to load the active metal target with the optimum amount of one hydrogen isotope, fill the tube with another hydrogen isotope at optimum pressure, and have these conditions remain static until the tube is operated.

Referring now to the vacuum-type tube disclosed in FIG. 2, it will be understood that this tube has a vacuumtight envelope comprising the same lower structure as shown in FIG. 1 including cylindrical sidewall 12, and

elements

11, 16, 17, 18 and 21. The two tubes differ only at their upper ends where in FIG. 2 a cup-shaped metal member 10 replaces the electrode 10 of FIG. 1. Member 10 is provided with a flange 13 which is metallically bonded to the upper end of cylinder 12, and a ceramic backing ring 14' is bonded to the upper surface of flange 13 to reinforce the seal. The end of member 10' is apertured to receive a tube of insulating material such as ceramic having a rim portion 26. The top and bottom surfaces of rim 26 are provided with annular

metallic coatings

27 and 28, respectively. Coating 28 is brazed to the end of member 10, and a ceramic backing ring 30 is brazed to the inside of member 10 to reinforce the seal. A metal tubulation 19 is brazed to the upper metallic coating 27 on rim 26, and a ceraMic backing ring 31 is brazed on the tubulation for reinforcement. Ceramic tube 25 is coated with two spaced metal strips 33 and 34 of active metal such as titanium, zirconium or hafnium. The oxide or other protective coating as described in connection with FIG. I is represented by 35 on

active metal layers

33 and 34 in FIG. 2. A narrow strip of metallizing 36 connects active metal 34 to the metallizing layer 27 and thus to tubulation I9. Similarly, a narrow strip of metallizing 37 connects active metal 33 to the metallizing layer 28 and thus to member 10'. It should be understood that

active metal layers

33 and 34 are also narrow strips extending along the ceramic tube 25 but not around it.

The vacuum-type tube described in connection with FIG. 2 is processed in exactly the same manner described in connection with the tube of FIG. 1 except that instead of introducing a hydrogen isotope gas after the active metal surfaces have been protected by the impervious coatings 21 and 35, the tube is evacuated and then sealed by pinching-off tubulation 19. It should be understood that metal layers 18, 34, and 35 are the only active metals in the tubes of FIGS. 1 and 2, all the other metal parts being nonactive metals.

In order to operate the tube of FIG. 2, an external power source (not shown) is connected to member 10'. the portion of 19 remaining after pinch-off, and electrode II. A potential difference can thus be obtained across

active metal layers

33 and 34 to cause a spark between them, which spark will destroy the coating 35 causing desorption of the hydrogen isotope in the

layers

33 and 34. The spark will also cause ionization of the desorbed isotope. As in the case of FIG. 1, the target 11 of the vacuum-type tube is negative so that ions will strike it to cause the neutron yielding reaction. Although it is preferred to have the spark between two loaded

metal areas

33 and 34, it will be understood that the

nonactive metallizing strip

36 or 37 could be extended to replace

area

34 or 33 so that only one of the spark electrodes would be a loaded active metal.

Having thus described the invention, what is claimed as new and desired to be securedby Letters Patent is:

l. The method of making a neutron source tube having an envelope containing an active metal in which is sorbed a hydrogen isotope, said method comprising the steps ofsorbing the isotope in the active metal by filling the envelope with the hydrogen isotope in the form of gas, thermally shocking said active metal while said gas is present in the envelope, then removing the hydrogen isotope gas, introducing into the envelope a coating gas chosen from the group consisting of oxygen, nitrogen, chlorine, and carbon, heating said active metal and coating gas to a temperature sufficient to cause said coating gas to react with said active metal and form a coating thereon which prevents desorption of the hydrogen isotope in the active metal, removing said coating gas, obtaining another gas content in the envelope, and sealing said envelope.

2. The method of processing an active metal to provide a target for a neutron source tube comprising the steps of exposing the active metal to a gas selected from the group consisting of deuterium and tritium, sorbing said gas in said active metal, and then reacting said active metal with a gas chosen from the group consisting of oxygen, nitrogen, chlorine, and carbon to form a coating on said active metal to seal in the sorbed gas, said reacting step being performed under conditions which prevent desorption ofsaid sorbed gas.

3. The method of making a target for neutron sources. said method comprising the steps of sorbing a gas selected from the group consisting of deuterium and tritium in an active metal and then sealing said active metal with a coating which prevents desorption of said gas and is penetrable by ions. said sealing step being performed under conditions which prevent desorption of said sorbed gas.

4. The method of claim 3 in which said sealing step coniprises exposing said active metal to oxygen gas at a pressure of 200 to 400 millimeters of mercury for about one hour at a temperature of to 200 C.

Claims

2. The method of processing an active metal to provide a target for a neutron source tube comprising the steps of exposing the active metal to a gas selected from the group consisting of deuterium and tritium, sorbing said gas in said active metal, and then reacting said active metal with a gas chosen from the group consisting of oxygen, nitrogen, chlorine, and carbon to form a coating on said active metal to seal in the sorbed gas, said reacting step being performed under conditions which prevent desorption of said sorbed gas.
3. The method of making a target for neutron sources, said method comprising the steps of sorbing a gas selected from the group consisting of deuterium and tritium in an active metal, and then sealing said active metal with a coating which prevents desorption of said gas and is penetrable by ions, said sealing step being performed under conditions which prevent desorption of said sorbed gas. 4. The method of claim 3 in which said sealing step comprises exposing said active metal to oxygen gas at a pressure of 200 to 400 millimeters of mercury for about one hour at a temperature of 150* to 200* C.